Optical densitometers are in wide use in the graphic arts industry to obtain measurements of relative light intensity reflected from, or transmitted through, a selected surface or portion of a surface. Measurments may be taken from photographs, photographic negatives, or other image source and the measurements are used to control subsequent photographic exposures. The standard scale in the photographic industry for optical density measurements has a logarithmic base and the present invention includes improved circuitry for converting linear light intensity inputs to the industry standard logarithmic scale.
The standard scale for optical density used in the graphic arts industry is a scale ranging from 0.00 to 4.00. Because of the logarithmic nature of this scale, optical density variations in the range of 1:10,000 may be accommodated. The standard logarithmic scale was developed in the Nineteenth Century when it was discovered that an exponential relationship existed between the mass of metallic silver in a developed photographic negative and the opacity of the negative to light transmission. As the silver mass per unit area increased linearly, opacity increased exponentially. It was also discovered that a linear relationship existed between the opacity of the developed image and the light exposure. Thus, a quantitative determination of the mass of silver present in the developed image provided a measurement of light exposure, the silver mass being logarithmically related to the light exposure.
Electronic instruments are now used to measure optical densities, in lieu of quantitative chemical analyses, but the original scale relationships have been maintained. Thus, "opacity" is still defined as the reciprocal of the percentage light transmission of a given sample and "optical density" is defined as the common logarithm of opacity. Input light intensities may be obtained by either light transmission through a given sample or light reflecting from a sample. The above definitions apply in either situation.
It is conventional to use photomultiplier tubes in electronic optical densitometers for converting input light intensities to electrical signals. The amplification factor of a photomultiplier tube is, among other factors, related to the voltage applied across the dynode system of the photomultiplier tube. The electron flow is increased by the same factor at each dynode so that the final amplification factor may be quite large. Conventional photomultiplier tubes used in the graphic arts industry thus require large voltages for operating the dynode system and obtain anode voltages which may be in the neighborhood of 500-1,000 volts.
In one prior art optical densitometer described in U.S. Pat. No. 3,765,776 to Bravenec, a resistor-capacitor (RC) network is used to provide an exponentially decaying voltage across the dynode system. Thus, the lower the input light intensity, the higher the dynode voltage at which a given anode current is obtained. This feature is used to trigger a counter, initiating a count cycle which terminates when the dynode voltage has discharged to a predetermined level. In this system, maximum dynode voltage is always supplied across the photomultiplier tube. The resulting large voltage swings are detrimental to the photomultiplier tube and to the associated circuit components.
In the prior art, signals generated by the photomultiplier tube in combination with an RC circuit are generally compared with internal reference signals, to produce outputs which may be used to actuate counting apparatus for a time period functionally related to the logarithm of the input light intensity. The need to match the photomultiplier tube and the RC circuit characteristics with internal references has required that photomultiplier tubes for use in optical densitometers having operating characteristics within a very narrow range. The production yield of tubes having such a narrow range of parameters is quite small and it would be very desirable to accommodate a wider range of photomultiplier tube characteristics.
Generally, standard optical density samples are available in the graphic arts industry for calibrating optical densitometers. The basic reference sample is the "white" sample which produces a scale output reading of 0.00. That is, the "white" sample produces a relative light intensity of 1. At the other end of the scale, a "dark" sample produces a relative light intensity of 1.times.10.sup.-4, or a relative opacity of 1.times.10.sup.4.
According to one aspect of the present invention, the operating parameters of the photomultiplier tube are set during an "automatic-zero" cycle where light from a "white" optical density sample is input to the photomultiplier tube. A reference dynode voltage is obtained, the corresponding anode current is determined, and a reference signal is derived corresponding to the anode current. The reference signal is then retained in the optical densitometer to maintain a constant current during a subsequent measurement cycle. In most instances, the reference condition is set using a "white" sample having an optical density of 0.00. In this region, it is very desirable to provide increased sensitivity in the automatic zeroing circuitry to obtain an accurate and stable reference anode signal. The disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for obtaining accurate optical density measurements.